U.S. Ambassador Lewis Tambs and at least two CIA officials in Central America assisted private efforts to equip the Nicaraguan resistance with weapons at a time when the U.S. government was strictly prohibited by Congress from providing military aid, a key operative in the supply network testified Tuesday.

Robert W. Owen, who served as a go-between with the rebels for then-White House aide Oliver L. North, provided the Senate and House committees investigating the Iran- contra affair with the first direct testimony that U.S. officials in Central America were routinely violating the ban on U.S. military assistance to the contras in 1985 and 1986.

Cites Help on Airstrip

Among other things, Owen said, Tambs, then the U.S. ambassador to Costa Rica, and a senior CIA official known as Tomas Castillo helped him select an airstrip site in Costa Rica for the supply network. In addition, he said that the CIA once arranged for the contra suppliers to use a DC-4 plane that previously had been involved in drug smuggling.

Likewise, Owen said he had reason to suspect that the late CIA Director William J. Casey and Alan D. Fiers, the CIA's Central American task force chief in Washington, had assisted North in the development of an alternative contra supply network after direct U.S. military aid was suspended.

Owen also recounted three highly secretive trips he made to New York City in 1985 at North's request to pick up large sums of money--including one bizarre mission that netted $9,500 in cash from a Chinese grocer. Committee members said that this may represent a new source of funds not previously uncovered by congressional investigators.

Following instructions from retired Air Force Maj. Gen. Richard V. Secord, Owen said he went to a Chinese grocery on the lower West Side of Manhattan, asked for a man whose name he had been given and told the man that "Mooey" sent him. In response, he said, the man walked behind a counter, bent down and pulled a wad of 95 $100 bills from his pants leg.

When he gave the money to Secord, he said he expressed his opinion that the money supplier had probably taken a 5% cut from what must have been an original sum of $10,000. But Secord replied that the sum was intentionally less than $10,000 to circumvent laws applying to money laundering, Owen said.

While Owen could not remember the name of the New York City bank where he picked up two other envelopes that he believed contained cash, he said that it could have been the Republic Bank. He said that he took those envelopes directly to North in his White House office.

These three sums obtained in New York did not come from contra leader Adolfo Calero--unlike other traveler's checks that North kept in his White House safe, according to Owen. "I have no idea where they came from," he said. "I imagine that they may have come from a wire transfer from an overseas bank. I just don't know."

Discloses Use of Ledger

He also disclosed that North kept a ledger of all checks--a document that investigators say has never been found.

Owen recalled that he received his first assistance from a CIA official in Central America in August, 1985, when he went to Costa Rica to scout for locations for an airstrip to be used by the private contra-supply effort. He said that Castillo, the agency's senior official there, took him to a possible airstrip site, lent him a camera to photograph the spot and also helped him to devise a "cover story" that the land would be used for agricultural experimentation.

At the time of these events, the CIA was strictly prohibited by Congress from providing any assistance to the Nicaraguan resistance, including intelligence information. The ban was enacted in 1984 in response to the CIA's role in mining a Nicaraguan harbor.

On the same trip, Owen said that he probably contacted Tambs about the airstrip as well as Castillo. "He (Tambs) knew what it was going in for," he added.

Sees Link to North

Owen said he later realized that Tambs and Castillo were working closely with North in developing the contra-supply network, particularly in an effort to bolster the so-called "southern front" of rebels fighting the Sandinista government's army.

During Owen's last conversation with Tambs in October, 1986, he recalled, the ambassador summed up his service in Costa Rica by saying: "Well, I came down here to try and help put together a southern front. That's what I was doing, that's what Mr. Castillo was doing, that's what you were doing and I think we did our best."

Owen's next reference to CIA assistance came five months later, after he was hired as a consultant to the State Department's Nicaraguan Humanitarian Assistance Office, which was set up to distribute non-military aid to the contras. As a $4,850-a-month government contractor, he said that he served primarily as North's "eyes and ears" at the humanitarian aid office.

Although the office was set up by Congress to dispense $27 million in so-called humanitarian aid to the contras, Owen said that it quickly became a tool of North's to further the military supply operation. The Times reported Tuesday that Airmach Inc.--the main air carrier employed by the agency--routinely ferried military equipment to the contras after its cargo planes entered the region carrying humanitarian goods supplied by the United States.

System Went Awry

The system went haywire on one occasion in March, 1986, when contra leader Calero apparently refused to release military goods under his control for use on the southern front, Owen said. After dropping humanitarian goods at the contras' air base in Honduras, he said, the humanitarian aid office's cargo plane flew to Ilopango air base in El Salvador to pick up a load of military goods. But there were no goods there waiting to be shipped.

Owen said that he learned of the problem before leaving Honduras and discussed it with an unnamed CIA official, who agreed to contact Castillo via a secure communications line. He received a message later that the equipment had not been released.

He said that he subsequently talked with Castillo directly by telephone from Ilopango but "that was more to tell him so that he could pass on the word to the freedom fighters who were in the jungles waiting for the drop, to let them know that the mission was not a go."

At Ilopango, he added, the contra-supply group met to discuss the problem with Col. Jim Steele, the commander of U.S. advisers in El Salvador, attending. According to Owen, Steele was there only as "an observer," although he later agreed to place a call on a secure line to CIA officials in Honduras.

Offers No Details

Owen offered no details of what he alleged to be the CIA's apparent role in hiring a cargo plane that had previously been involved in drug trafficking. He said that it was arranged by a Mr. Foley of Summit Aviation in Delaware, a firm sometimes used by the CIA.

In a memo dated Feb. 10, 1986, Owen told North: "No doubt you know the DC-4 Foley got was used at one time to run drugs and part of the crew had criminal records. Nice group 'the Boys' chose. The company is also one that Mario (Calero's brother) has been involved with using in the past, only they had a quick name change. Incompetence reins."

When asked by Senate committee Chairman Daniel K. Inouye (D-Hawaii) to identify "the boys" to whom he referred in his memo, Owen replied: "The CIA."

Owen said he suspected that North was getting help from CIA task force chief Fiers because he overheard one telephone conversation between the two. He said he believed that classified maps North gave to the contras may have come directly from Fiers.

Last December, according to Sen. David L. Boren (D-Okla.), Fiers told the Senate Intelligence Committee in closed hearings that he had turned down a request from North to prepare intelligence for delivery to the contras.

Underlines Casey, North Ties

It has already been established by previous witnesses that Casey had a close relationship with North. Owen said that North frequently received calls from Casey while he was visiting the White House aide, and he once encountered the CIA chief coming out of North's office as he was going in.

Owen, who worked for the humanitarian aid office from October, 1985, through May, 1986, said he was hired on the recommendation of a mid-level Administration group that included North, Fiers and Assistant Secretary of State Elliott Abrams--against the wishes of Bruce Deumling, who headed the agency.

Even though his contract prohibited him from providing military assistance to the contras, Owen said that he undertook his role in North's network as an after-hours project. He said that he was told by North: "Look, you're doing this for me, you're on your own time."